Somitogenesis consists of the formation, segmentation, and differentiation of the paraxial mesoderm. This sequence of events begin sat gastrulation and continues through organogenesis to add somites to the lengthening posterior axis. The overall goal of this grant proposal is to investigate the cell behaviors and signaling mechanisms that underlie somitogenesis in the frog, Xenopus/aevis. Since most of what is known about somitogenesis comes from direct-developing vertebrate models such as chick and mice, this research will contribute significantly to our understanding of this process in in-direct developers such as Xenopus. An innovative approach consisting of microscopy, microsurgery, transplantation, and molecular techniques will be used to examine somitogenesis in Xenopus. The first specificaim will examine cell behaviors such as cell shape changes, protrusive cell activity, and cell movements involved in paraxial mesoderm formation, segmentation and rotation. Once normal cell behaviors are understood, then the cell behaviors in embryos that undergo abnormal fissure formation and somite formation will be examined. These results will elucidate how normal cell behaviors are coordinated to form organized somites. The second specific aim will examine whether somitogenesis involves intrinsic or extrinsic signals. Previous work from our lab has revealed that somite-inducing signals are present in the posterior paraxial mesoderm of tailbud embryos to instruct mesodermal cells to differentiate into myotome. To extend this study, transplantation experiments will be performed in which competent cells are grafted to more anterior regions of the paraxial mesoderm. These results will reveal whether somite inducing signals are active throughout the paraxial mesoderm or localized to the posterior paraxial mesoderm. Unlike most vertebrates, Xenopus somites give rise to primarily myotome cells. To examine this issue, the third specificaim consists of using a vital dye fate mapping approach to determine the mesodermal origins of the dermatome and sclerotome. Next, it will be determined whether axial structures play a role in somite differentiation as previously found in other vertebrate model organisms. Together, this research will provide undergraduate and graduate students from under-represented groupsat SFSU a unique opportunity to learn essential skills that will prepare them for future careers in biomedical research.